Kinetic analysis of the chloride dependence of the neuronal uptake of dopamine and effect of anions on the ability of substrates to compete with the binding of the dopamine uptake inhibitor GBR 12783

Interaction of catechol and non-catechol substrates with externally or internally facing dopamine transporters

Our previous work suggested that collapsing the Na(+) gradient and membrane potential converts the dopamine (DA) transporter (DAT) to an inward-facing conformation with a different substrate binding profile. Here, DAT expressing human embryonic kidney 293 cells were permeabilized with digitonin, disrupting ion/voltage gradients and allowing passage of DAT substrates. The potency of p-tyramine and other non-catechols (d-amphetamine, beta-phenethylamine, MPP(+)) in inhibiting cocaine analog binding to DAT in digitonin-treated cells was markedly weakened to a level similar to that observed in cell-free membranes. In contrast, the potency of DA and another catechol, norepinephrine, was not significantly changed by the same treatment, whereas epinephrine showed only a modest reduction. These findings suggest that catechol substrates interact symmetrically with both sides of DAT and non-catechol substrates, favoring binding to outward-facing transporter. In the cocaine analog binding assay, the mutant W84L displayed enhanced intrinsic binding affinity for substrates in interacting with both outward- and inward-facing states; D313N showed wild-type-like symmetric binding; but D267L and E428Q showed an apparent improvement in the permeation pathway from the external face towards the substrate site. Thus, the structure of both substrate and transporter play a role in the sidedness and mode of interaction between them.

The importance of company: Na+ and Cl- influence substrate interaction with SLC6 transporters and other proteins

SLC6 transporters, which include transporters for gamma-aminobutyric acid (GABA), norepinephrine, dopamine, serotonin, glycine, taurine, L-proline, creatine, betaine, and neutral cationic amino acids, require Na+ and Cl- for their function, and this review covers the interaction between transporters of this family with Na+ and Cl- from a structure-function standpoint. Because detailed structure-function information regarding ion interactions with SLC6 transporters is limited, we cover other proteins cotransporting Na+ or Cl- with substrate (SLClA2, PutP, SLC5A1, melB), or ion binding to proteins in general (rhodanese, ATPase, LacY, thermolysine, angiotensin-converting enzyme, halorhodopsin, CFTR). Residues can be involved in directly binding Na+ or Cl-, in coupling ion binding to conformational changes in transporter, in coupling Na+ or Cl- movement to transport, or in conferring ion selectivity. Coordination of ions can involve a number of residues, and portions of the substrate and coupling ion binding sites can be distal in space in the tertiary structure of the transporter, with other portions that are close in space thought to be crucial for the coupling process. The reactivity with methanethiosulfonate reagents of cysteines placed in strategic positions in the transporter provides a readout for conformational changes upon ion or substrate binding. More work is needed to establish the relationships between ion interactions and oligomerization of SLC6 transporters.

Interactions of cations and anions with the binding of uptake blockers to the dopamine transporter

Uptake blockers and substrates are likely to recognise a common binding domain on the dopamine neuronal transporter (DAT). Among cations that form ionic gradients at the level of the cellular plasma membrane, Na+ is the only one that can stimulate their binding. The binding stimulation appears over Na+ concentrations ranging from 0 to 10-60 mM; at higher Na+ concentrations, binding reaches a plateau or decreases, according to the uptake blocker that is studied. The majority of the other cations, including K+, Ca2+, Mg2+ and Tris+, inhibit the binding of uptake blockers. Several metals impair binding to the DAT and/or the dopamine transport, but, under specific conditions, some of them, and chiefly Zn2+, stimulate binding. The complex relationships between cations, uptake blockers and the DAT suggest that cations recognise at least three different sites: the first one, site 1, is for cation-induced binding inhibition; the second one, site 2, is for Na+-induced binding stimulation; and the third one, site 3, is for Zn2+-induced binding stimulation. Modelling of the interactions between Na+, K+ and radioligands allows a better understanding of the effects of cations at sites 1 and 2, and of uptake blockers at site 1. Some anions also facilitate the binding of uptake blockers to the DAT, as far as they are associated with Na+. The dependence of the binding of dopamine on ions could be involved in its preferential inward transport and used by uptake blockers for their own binding to the DAT.

Binding of cocaine-like radioligands to the dopamine transporter at 37°C: effect of Na+ and substrates

Although dopamine (DA) translocation by the DA transporter (DAT) requires Na+, a role for Na+ in the DA recognition step in the translocation cycle has been questioned. Thus, when binding techniques were used to indirectly measure the affinity of DA for DAT via its potency in inhibiting cocaine analog binding, no stimulation of DA binding was observed when assay temperature was at or below room temperature. The present work describes the use of 3H-labeled cocaine analogs for assays at 37 degrees C. When there is sufficient Na+ in the medium (> or =25 mM), [3H]2beta-carbomethoxy-3beta-(4-iodophenyl)tropane ([3H]CIT) is an excellent radioligand to label human DAT with high affinity in membrane preparations of HEK-293 cells expressing the transporter. However, at 0 and 5 mM of Na+, appreciable binding of [3H]CIT occurs to proteins other than DAT, hampering accurate assessment of DAT-associated binding. No such problems occur with the binding of the 4-fluoro homolog of [3H]CIT, [3H]CFT at 37 degrees C, and this radioligand can be used at low [Na+], provided enough protein is present in the assay. The application of these assays show that, in contrast to the strong Na+ dependency of the binding of CFT, the substrates DA, D-amphetamine, p-tyramine, and DL-octopamine are not stimulated by Na+. This demonstrates that lack of Na+ stimulation of binding of substrates, including DA to DAT, in membrane preparations at room temperature is not caused by the reduced fluidity of the frozen state of the hydrocarbon membrane interior at this temperature as compared with the liquid-expanded state at 37 degrees C.

Rapid regulation of dopamine transporter function by substrates, blockers and presynaptic receptor ligands

The extracellular actions of dopamine are terminated primarily through its binding to dopamine transporters and translocation back into dopamine neurons. The transporter thereby serves as an optimal target to regulate dopamine neurotransmission. Although acute pharmacological blockade of dopamine transporters is known to reversibly inhibit transporter function by preventing the binding of its endogenous substrate dopamine, it recently has become clear that dopamine transporter substrates, such as amphetamines, and blockers, such as cocaine, also have the ability to rapidly and persistently regulate transporter function after their direct pharmacological effect has subsided. Presynaptic receptor ligands can also regulate dopamine transporter function. This has been investigated most extensively for dopamine D2 receptors, but there is also evidence for regulation by gamma-aminobutyric acid (GABA) GABAB receptors, metabotropic glutamate, nicotinic acetylcholine, serotonin, sigma2- and kappa-opioid receptors. The focus of this review is the rapid, typically reversible, regulation of dopamine transporter velocity by substrates, blockers and presynaptic receptor ligands. The research discussed here suggests that a common mechanism through which these different classes of compounds regulate transporter activity is by altering the cell surface expression of dopamine transporters.

Na+ and the substrate permeation pathway in dopamine transporters

Advances have been made in characterizing the relationship between Na+ and the substrate permeation pathway in the dopamine transporter. This review covers the role of Na+ in co-transport with dopamine as well as in the recognition of dopamine. Apparent recognition depends on the preparation studied: it differs between intact cells heterologously expressing the dopamine transporter and membranes prepared from these cells. In our search for amino acid residues in the transporter involved in Na+ action, W84 and D313 were found to play a special role in cation interaction, with evidence for regulation of both Na+ and H+ sensitivity. Mutation of D313 to N appeared to decrease the affinity for the dopamine transporter in intact cells, not by altering recognition per se. A model is proposed in which access of dopamine, not recognition itself, is regulated by D313 and Na+. Thus, the role of external Na+ in intact cell preparations is to turn dopamine transporters to the externally facing form, allowing access of dopamine to its binding site.

The functioning neuronal transporter for dopamine: kinetic mechanisms and effects of amphetamines, cocaine and methylphenidate

The dopamine transporter (DAT) is a transmembrane spanning protein that catalyzes the transport of dopamine across the neuronal membrane to concentrate the neurotransmitter inside the cell. Although the uptake of dopamine has been studied since the 1960s, more recent advances in knowledge of the protein itself and in making kinetically resolved measurements of its action have led to more insights into its mechanism and pharmacology. The literature of the kinetics of transporters and kinetic measurements of DAT activity is reviewed to provide an overview of the multisubstrate mechanism of DAT activity, its pharmacology with regard to amphetamine, cocaine and methylphenidate, and correlations of DAT activity with some behavioral outputs.

Impairment of hippocampal CA1 heterosynaptic transformation and spatial memory by beta-amyloid(25-35)

In Alzheimer's disease, the cholinergic damage (reduced neurotransmission) and cognitive impairment occur long before beta-amyloid (Abeta) plaque formation. It has not been established whether the link between soluble Abeta and cholinergic functions contributes to synaptic dysfunction that underlies the cognitive impairment. Here, we report that Abeta(25-35), an active form of Abeta, inhibited long-term synaptic modification that depends on the associative activation of cholinergic and GABAergic inputs when bilaterally injected intracerebroventricularly (icv; 200 microg/site). The Abeta microinjections did not affect single-pulse-evoked glutamatergic and GABAergic synaptic transmission onto the hippocampal CA1 pyramidal cells, while cholinergic intracellular theta; was dramatically reduced by the Abeta(25-35) injection. Spatial memory of the water maze task was also impaired by the bilateral icv Abeta(25-35) injections, while bilateral microinjections of the same dose of Abeta(35-25) was ineffective in affecting the long-term synaptic modification evoked by associative activation of cholinergic and GABAergic inputs, the cholinergic intracellular theta;, or producing memory impairments. Thus restoring the synaptic plasticity involved in this associative activation of cholinergic and GABAergic inputs may offer an important therapeutic target in the treatment of early Abeta-induced memory decline.

Involvement of the NH2 terminal domain of catecholamine transporters in the Na(2+) and Cl(-)-dependence of a [3H]-dopamine uptake

The ionic dependence of the [3H]-dopamine uptake was studied in transfected cells expressing the human neuronal transporter for dopamine (hDAT) or noradrenaline (hNET), and chimeric transporters resulting from the symmetrical exchange of the region from the NH2 terminal through the first two transmembrane domains (cassette I). Chimera A is formed by hDAT comprising cassette I from hNET, whereas chimera B corresponds to the reverse construct. The appearance or the intensity of a Cl(-)-independent component of transport was linked to the presence of the COOH terminal part of hNET in both monoclonal and polyclonal Ltk(-) cells (Cl(-) substituted by isethionate and NO3(-), respectively), and in transiently transfected COS-7 cells. Cassette I was also involved in the Cl(-)-dependence because the transport activity of polyclonal Ltk(-) cells expressing A was partly Cl(-)-independent and because Ltk(-) cells expressing transporters containing cassette I of hDAT displayed higher K(mCl)- values than cells expressing the reverse constructs. In monoclonal Ltk(-) cell lines, K(mNa)+ values and biphasic vs monophasic dependence upon Na(+) concentrations differentiate transporters containing cassette I of hNET from those containing cassette I of hDAT. In COS-7 cells, the exchange of cassette I produced a significant change in Hill number values. In Na(+)-dependence studies, exchange of the COOH terminal part significantly modified Hill number values in both Ltk(-) and COS-7 cells. Hill number values close to two were found for hNET and hDAT when sucrose was used as substitute for NaCl. The NH2 terminal part of the transporters bears some of the differences in the Na(+) and Cl(-)-dependence of the uptake that are observed between hDAT and hNET. Present results also support a role of the COOH terminal part in the ionic dependence.

Differential sensitivity to NaCl for inhibitors and substrates that recognize mutually exclusive binding sites on the neuronal transporter of dopamine in rat striatal membranes

Addition of NaCl (90--290 mM) to a 10 mM Na(+) medium did not significantly modify B(max) and K(d) values for [3H]mazindol binding to the dopamine neuronal transporter (DAT) studied on rat striatal membranes at 20 degrees C. Addition of NaCl differentially affected the ability of other uptake inhibitors and substrates to block the [3H]mazindol binding. Ratios of 50% inhibiting concentrations calculated for 290 and 90 mM NaCl allowed to distinguish three groups of agents: substrates which were more potent in the presence of 290 mM NaCl (group 1; ratio < 1) and two groups of uptake inhibitors displaying ratio values either ranging around two (group 2: WIN 35,428, cocaine, methylphenidate, pyrovalerone) or close to unity (group 3: BTCP, mazindol, benztropine, nomifensine). However, agents from these three groups recognize mutually exclusive binding sites since in interaction studies the presence of WIN 35,428 (group 2) or mazindol (group 3) increased the 50% inhibiting concentrations of D-amphetamine (group 1) and WIN 35,428 on the [3H]mazindol binding to theoretical values expected for a competition of all of these compounds for the same binding domain on the DAT.

Structural domains of chimeric dopamine-noradrenaline human transporters involved in the Na(+)- and Cl(-)-dependence of dopamine transport

Catecholamine transporters constitute the biological targets for several important drugs, including antidepressants, cocaine, and related compounds. Some information exists about discrete domains of these transporters that are involved in substrate translocation and uptake blockade, but delineation of domains mediating the ionic dependence of the transport remains to be defined. In the present study, human neuronal transporters for dopamine and noradrenaline (hDAT and hNET) and a series of six functional chimeras were transiently expressed in LLC-PK1 cells. Substitution of Cl(-) by isethionate reveals that cassette IV (i.e., the region of the transporter encompassing transmembrane domain 9 through the COOH terminal) plays an important role in the Cl(-)- dependence of the uptake. Substitutions of Na(+) and NaCl by Tris(+) and sucrose, respectively, demonstrate that three different segments scattered across the transporter are involved in the Na(+)- dependence of the transport activity: cassette I (i.e., the region from the amino terminus through the first two transmembrane domains), cassette IV, and junction between transmembrane domains 3 to 5 and 6 to 8. Results of the present work also suggest that the use of Tris(+) as a substitute for Na(+) results in a biased estimate of the Hill number value for hDAT. This study provides useful clues for identifying specific residues involved in the uptake function of the catecholamine transporters.

Interaction of Na+, K+, and Cl- with the binding of amphetamine, octopamine, and tyramine to the human dopamine transporter

Little information is available on the role of Na+, K+, and Cl- in the initial event of uptake of substrates by the dopamine transporter, i.e., the recognition step. In this study, substrate recognition was studied via the inhibition of binding of [3H]WIN 35,428 [2beta-carbomethoxy-3beta-(4-fluorophenyl)[3H]tropane], a cocaine analogue, to the human dopamine transporter in human embryonic kidney 293 cells. D-Amphetamine was the most potent inhibitor, followed by p-tyramine and, finally, dl-octopamine; respective affinities at 150 mM Na+ and 140 mM Cl- were 5.5, 26, and 220 microM. For each substrate, the decrease in the affinity with increasing [K+] could be fitted to a competitive model involving the same inhibitory cation site (site 1) overlapping with the substrate domain as reported by us previously for dopamine. K+ binds to this site with an apparent affinity, averaged across substrates, of 9, 24, 66, 99, and 134 mM at 2, 10, 60, 150, and 300 mM Na+, respectively. In general, increasing [Na+] attenuated the inhibitory effect of K+ in a manner that deviated from linearity, which could be modeled by a distal site for Na+, linked to site 1 by negative allosterism. The presence of Cl- did not affect the binding of K+ to site 1. Models assuming low binding of substrate in the absence of Na+ did not provide fits as good as models in which substrate binds in the absence of Na+ with appreciable affinity. The binding of dl-octopamine and p-tyramine was strongly inhibited by Na+, and stimulated by Cl- only at high [Na+] (300 mM), consonant with a stimulatory action of Cl- occurring through Na+ disinhibition.

Multisubtrate mechanism for the inward transport of dopamine by the human dopamine transporter expressed in HEK cells and its inhibition by cocaine

Rotating disk electrode voltammetry was used to measure the time-resolved inward transport of dopamine into human embryonic kidney cells expressing the human transporter for dopamine and a kinetic mechanism of transport is hypothesized. Dopamine transport in this preparation was highly concentrative, with a 10(6)-10(7) inward bias, first order in dopamine and the K(m) and V(max) were found to be 1.6 microM and 18 pmol/sec x 10(6) cells), respectively. The hDAT turnover was estimated to be approximately 18 s(-1) and the second order rate constant of association of dopamine with hDAT was approximately 10(7) M(-1)s(-1). Dopamine transport was found to have a second order dependence on Na(+) (K(Na) approximately 100 mM) and a first order dependence on Cl(-) (K(Cl) approximately 12 mM). Multisubstrate analyses suggested that hDAT operates with an ordered kinetic mechanism in which Na(+) binds first to the transporter protein, dopamine second, and Cl(-) last before translocation of dopamine into or across the membrane. Cocaine competitively inhibited dopamine transport (reaction order of unity and K(i) approximately 0.34 microM) with no discernible effect at the Na(+) and Cl(-) binding sites. These results differ from those of previous studies conducted in preparations of the striatum and nucleus accumbens. Comparisons of the variant results are made and an analysis of the differing apparent kinetic mechanisms is presented.

Modeling of the interaction of Na+ and K+ with the binding of dopamine and [3H]WIN 35,428 to the human dopamine transporter

Although much is known about the effects of Na+, K+, and Cl- on the functional activity of the neuronal dopamine transporter, little information is available on their role in the initial event in dopamine uptake, i.e., the recognition step. This was addressed here by studying the inhibition by dopamine of the binding of [3H]WIN 35,428 [2beta-carbomethoxy-3beta-(4-fluorophenyl)[3H]tropane], a phenyltropane analogue of cocaine, to the cloned human dopamine transporter expressed in HEK-293 cells. The decrease in the affinity of dopamine (or WIN 35,428) binding affinity with increasing [K+] could be fitted to a competitive model involving an inhibitory cation site (1) overlapping with the dopamine (or WIN 35,428) domain. The K+ IC50 for inhibiting dopamine or WIN 35,428 binding increased linearly with [Na+], indicating a K(D,Na+) of 30-44 mM and a K(D,K+) of 13-16 mM for this cation site. A second Na+ site (2), distal from the WIN 35,428 domain but linked by positive allosterism, was indicated by model fitting of the WIN 35,428 binding affinities as a function of [Na+]. No strong evidence for this second site was obtained for dopamine binding in the absence or presence of low (20 mM) Cl- and could not be acquired for high [Cl-] because of the lack of a suitable substitute ion for Na+. The K(D) but not Bmax of [3H]WIN 35,428 binding increased as a function of the [K+]/[Na+] ratio regardless of total [Cl-] or ion tonicity. A similar plot was obtained for the Ki of dopamine binding, with Cl- at > or = 140 mM decreasing the Ki. At 290 mM Cl- and 300 mM Na+ the potency of K+ in inhibiting dopamine binding was enhanced as compared with the absence of Cl- in contrast to the lack of effect of Cl- up to 140 mM (Na up to 150 mM). The results indicate that Cl- at its extracellular level enhances dopamine binding through a mechanism not involving site 1. The observed correspondence between the WIN 35,428 and dopamine domains in their inclusion of the inhibitory cation site explains why many of the previously reported interrelated effects of Na+ and K+ on the binding site of radiolabeled blockers to the dopamine transporter are applicable to dopamine uptake in which dopamine recognition is the first step.

Which form of dopamine is the substrate for the human dopamine transporter: the cationic or the uncharged species?

The question of which is the active form of dopamine for the neuronal dopamine transporter is addressed in HEK-293 cells expressing the human dopamine transporter. The Km value for [3H]dopamine uptake fell sharply when the pH was increased from 6.0 to 7.4 and then changed less between pH 7.4 and 8.2. The KI for dopamine in inhibiting the cocaine analog [3H]2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane binding displayed an identical pH dependence, suggesting that changes in uptake result from changes in dopamine recognition. Dopamine can exist in the anionic, neutral, cationic, or zwitterionic form, and the contribution of each form was calculated. The contribution of the anion is extremely low (</=0.1%), and its pH dependence differs radically from that of dopamine binding. The increase in the neutral form upon raising the pH can model the results only when the pKa1 (equilibrium neutral-charged) is set to a much lower value (6.8) than reported for dopamine in solution (8.86). The sum of cationic and zwitterionic dopamine concentrations remained constant over the entire pH range studied. These forms are the likely transporter substrates with pH-dependent changes occurring in their interaction with the transporter. The binding of dopamine, a hydroxylated phenylethylamine derivative, displays the same pH dependence as guanethidine, a heptamethyleniminoethyl- guanidine derivative fully protonated under our conditions. An ionizable residue in the transporter could be involved that does not interact with or impact the binding of bretylium, a quaternary ammonium phenylmethylamine derivative that is always positively charged and shows only a minor reduction in KI upon increasing pH.

Mechanistic analyses of ion dependences in a high-affinity human serotonin transport system in transfected murine fibroblast cells

1. A clonal cell line, L-S1, has been identified from transfection of human genomic DNA into cultured mouse L-M fibroblasts. Because this transfectant cell line stably expresses a high-affinity serotonin (5-HT) transport mechanism with kinetic and pharmacological properties comparable to those of other serotonin uptake systems, it was used to investigate the mechanistic involvement of Na+ and Cl- ions in the ligand binding and kinetic uptake processes of this system. 2. Intact transfectant cells, when incubated at low temperature (4 C), enabled quantitative assessment of imipramine-displaceable 5-[3H]HT binding to the 5-HT transport system. This binding activity is insensitive to the presence of various ligands specific for 5-HT receptor subtypes. 3. Imipramine-displaceable 5-[3H]HT binding to intact L-S1 cells was shown to be a Cl--dependent but Na+-independent process. Chloride ions lack binding co-operativity in facilitating ligand binding. Changes in external Cl- concentration altered the Kd but not the Bmax of binding. 4. The overall transport activity was observed to be highly dependent on both external Na+ and Cl- concentrations, characterized by a 5-HT:Na+:Cl- coupling ratio of 1:1:1 per transport cycle. Alterations in the external concentrations of both Na+ and Cl- ions altered only the Km and not the Vmax of transport. 5. Both binding and kinetic results are consistent with kinetic modelling predictions of the Cl- ion in facilitating 5-HT binding to the transport system, and of the Na+ ion in enabling translocation of bound 5-HT across the plasma membrane. Thus, Na+ and Cl- ions facilitate mechanistically distinct and discernible functions in the transport cycle.

Cation requirements of basal and ATP-regulated dopamine transport in rat pheochromocytoma cells

The transport of dopamine into presynaptic nerve terminals is the primary mechanism for the termination of dopaminergic neurotransmission. This transport process has recently been found to be composed of two components, a basal dopamine transport pathway which exists in the absence of extracellular ATP and an ATP-regulated moiety which comprises approximately 66% of the total transport system [Cao C. J. et al. (1990) Biochem. Pharmac. 39, R9-R14; Cao C. J. et al. (1989) Biochemistry 8, 207-220; Dunigan C. D. and Shamoo A. E. (1995) Neuroscience 65, 1-4; Eshleman A. et al. (1995) Life Sci. 56, 1613-1621]. Using a rat pheochromocytoma cell line and a Krebs bicarbonate buffering system, the present study examined the effect of several cations on both basal and ATP-regulated dopamine transport. In the absence of extracellular ATP, dopamine transport had an absolute dependence on the presence of Na+, but exhibited no requirement for Mg2+. Kinetically, the addition of 120 mM NaCl increased the Vmax of basal dopamine transport by approximately 150%. In contrast, the ATP-regulated dopamine transport pathway displayed a different sensitivity to Na+ and was completely dependent upon the presence of Mg2+. The addition of 1.2 mM MgSO4 increased the Vmax of transport in the presence of 0.7 mM extracellular ATP by 222%. Both basal and ATP-regulated transport were unaffected by the removal of either Ca2+ or K+ from the assay buffer. When the effects of ouabain, a potent inhibitor of Na+, K(+)-ATPase, were tested in the rat pheochromocytoma cell model, it was found that concentrations of ouabain as high as 1 mM were ineffective at inhibiting either the basal or ATP-regulated dopamine transport components. These results imply that the Na+ gradient supplied by Na+, K(+)-ATPase is not the sole provider of energy needed to drive either transport process. The ionic requirements of the basal and ATP-regulated dopamine transport pathways demonstrate the distinction between the two transport processes. In addition, the ionic dependency profile of the ATP-regulated moiety has provided some mechanistic insights into ATP-regulated catecholamine uptake, as the absolute Mg2+ requirement and the ineffectiveness of Ca2+ argues against the involvement of either purinergic receptors or a Ca(2+)-dependent, Mg(2+)-independent ectokinase in the ATP-regulated transport system.

Effect of low concentrations of K+ and Cl- on the Na(+)-dependent neuronal uptake of [3H] dopamine

The specific uptake of [3H] dopamine (DA) was studied using a crude synaptosomal fraction obtained from rat striatum. In a medium containing a 10 mM NaHCO3/NaH2PO4 buffer and no added K+ ions, addition of NaCl elicited an increase in DA uptake for Na+ concentrations from 10 to 60 mM, and then a decrease of uptake for Na+ concentrations up to 130 mM. These data confirm that rather low NaCl concentrations produce a maximal DA uptake. This biphasic curve of uptake resulted from significant changes in the Vmax of the DA uptake. Except for 10 mM Na+, this curve was not significantly modified when 9 mM NaHCO3/NaH2PO4 were replaced by 9 mM NaCl. This result indicates that the Cl- dependence of the DA uptake is mainly secondary to the Na+ dependence. Addition of KCl up to 3 mM did not modify the ascending part of the NaCl-dependent uptake curve. In contrast, the reduction in uptake produced by high Na+ concentrations was prevented in a concentration-dependent manner by KCl; this effect resulted from a decrease in the Km and an increase in the Vmax for the uptake. Measurements of membrane potential, with the help of the fluorescent probe 3, 3'-diethylthiadicarbocyanine iodide [DiSC2(5)] and purified synaptosomes prepared from rat striatum and cerebral cortex, revealed that addition of 3 mM KCl to a medium containing a high Na+ concentration and no K+ ions produced a marked and stable decrease in the fluorescence level. This decrease which corresponds to an increase in membrane polarization was blocked by 0.1 mM ouabain. These data suggest that low K+ concentrations are likely to prevent the decrease in uptake elicited by high Na+ concentrations by restoration, via a Na+/K+ ATPase-mediated mechanism, of the membrane potential and/or a transmembrane electrochemical Na+ gradient more favourable to DA uptake.

Interaction of two sulfhydryl reagents with a cation recognition site on the neuronal dopamine carrier evidences small differences between [3H]GBR 12783 and [3H]cocaine binding sites

We have compared the effect of treating rat striatal cell membranes with ionic hydrophilic sulfhydryl reagents on the specific bindings of [3H]cocaine and of [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-3H]propenyl)-piperaz ine) to the neuronal transporter of dopamine. Treatment with 1 mmol/l 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) resulted in similar time- and concentration-dependent reductions of the specific binding of both radioligands. None of the uptake blockers tested afforded any protection against 1 mmol/l DTNB. Addition of (sub)millimolar concentrations of CaCl2 or MgCl2, or 250 mmol/l KCl to a treatment medium containing 10 mmol/l Na+ significantly increased the DTNB-induced reduction of the specific binding of both radioligands. Cations were likely to be responsible for this effect since ions in combination with DTNB induced similar reductions in binding when either 1 mmol/l CaCl2 or 50-250 mmol/NaCl were added. Effects of cations on the DTNB-induced inhibition of binding were generally more marked on [3H]GBR 12783 than on [3H]cocaine binding. When added to a medium containing 10 mmol/l Na+ 1 mmol/l DTNB induced a reduction in the Bmax of the specific binding of both radioligands. Addition of 1 mmol/l Ca2+ maintained or increased this Bmax reduction and elicited a decrease in affinity which was significant for [3H]GBR 12783 binding. Treatment of membranes with the sodium salt of p-hydroxymercurybenzenesulfonate (pHMBS) induced time- and concentration-dependent decreases in [3H]GBR 12783 binding which were significantly greater than decreases in [3H]cocaine binding. However, 50 mumol/l pHMBS produced a similar decrease in the Bmax of the specific binding of both radioligands. The pHMBS-induced reduction of [3H]GBR 12783 binding was not reversed by drugs whose action is purely that of uptake inhibition or by substrates of the dopamine carrier. Some of these drugs (100 mumol/l dopamine, 1 mumol/l mazindol or 100 mumol/l cocaine) protected the specific binding of [3H]cocaine against the effects of pHMBS, whereas 1 mmol/l p-tyramine, 10 mumol/l nomifensine and 10 nmol/l GBR 12783 were ineffective. Addition of 120 mmol/l Na+, 1 mmol/l Ca2+ or 10 mmol/l Mg2+ to a treatment medium containing 10 mmol/l Na+ significantly reduced the effects of pHMBS on the specific binding of both radioligands. When striatal cell membranes were treated in a medium containing 130 mmol/l Na+, there was a general decrease in the effects of ions on the reductions of specific binding produced by DTNB or pHMBS.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of CH3HgCl and several transition metals on the dopamine neuronal carrier; peculiar behaviour of Zn2+

CH3Hg+ and metal ions inhibited the specific binding of (1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-3H]propenyl) piperazine) ([3H]GBR 12783) to the dopamine neuronal carrier present in membranes from rat striatum with a general rank order of potency CH3Hg+ > Cu2+ > Cd2+ > Zn2+ > Ni2+ = Mn2+ = Co2+, suggesting that -SH groups are chiefly involved in this inhibition. Five millimolar dithiothreitol reversed the rather stable block of the specific binding produced by Cd2+ or Zn2+. An increase in the concentration of Na+, or addition of either K+ or Ca2+ reduced the inhibitory effects of metal cations, except Cu2+. Zn2+ (3 microM) reduced the inhibitory potency of Cd2+ on the binding but was ineffective against CH3Hg+ and Cu2+. Zn2+ at 0.3 to 10 microM significantly enhanced the specific binding of [3H]GBR 12783 and [3H]cocaine by 42 to 146%. Zn2+ (3 microM) increased the affinity of all pure uptake inhibitors tested and of the majority of the substrates for the [3H]GBR 12783 binding site. Dissociation experiments revealed that Zn2+ both inhibited and enhanced the [3H]GBR 12783 binding by recognizing amino acids located close to or in the radioligand binding site. Micromolar concentrations of Zn2+ noncompetitively blocked the [3H]dopamine uptake but they did not modify the block of the transport provoked by pure uptake inhibitors. These findings suggest that Na+, K+, Ca2+ and metal ions could recognize some -SH groups located in the [3H]GBR 12783 binding site; low concentrations of Zn2+ could allow a protection of these -SH groups.

Specific binding of [3H]GBR 12783 to the dopamine neuronal carrier included in polarized membranes

We have compared the properties of the binding to the neuronal dopamine carrier located either in polarized membranes of synaptosomes or in non polarized, classical membranes. Non-polarized membranes were prepared by sonication of the partially purified synaptosomal fraction obtained from rat striatum which was used as the source of polarized membranes. Binding experiments were carried out at 37 degrees C in Krebs Ringer related media. [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1- 3H]propenyl)piperazine) specifically bound with a nanomolar affinity to a homogeneous population of site (maximal binding site concentration: 8-10 pmol/mg protein). Pure uptake inhibitors, but not substrates, competed for the [3H]GBR 12783 binding site located in polarized membranes of synaptosomes at concentrations effective against dopamine neuronal transport. Except for [3H]GBR 12783, the replacement of Cl- by isethionate- did not result in significant change in the ability of pure uptake inhibitors to compete for the specific binding site. A reduction in the Na+ concentration from 135 to 10 mM induced a significant decrease in the inhibitory potency of GBR 12783, mazindol, nomifensine and methylphenidate. This decrease was likely to result from the presence of K+, Mg2+ and Ca2+, whose inhibitory effects were modified and/or increased by decreasing the Na+ concentration. These data indicate that the membrane polarity is not clearly involved in the binding of pure uptake inhibitors to the dopamine neuronal carrier; furthermore they underline the critical role of Na+ and K+ transmembrane gradients in both the recognition of the carrier by dopamine and its inward transport.

Cationic and anionic requirements for the binding of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)[3H]tropane to the dopamine uptake carrier

The present study reports the ion dependency of 2 beta-carbomethoxy-3 beta-(4-fluorophenyl)[3H]tropane ([3H]CFT) binding to the dopamine transporter in the rat striatum. The results indicate that [3H]CFT binding to synaptosomal P2 membranes requires low concentrations of Na+ (peak binding between 20 and 50 mM Na+), is stimulated by phosphate anion or I-, but is unaffected or only slightly affected by F-, Cl-, Br-, NO3-, or SO(4)2-. Concentrations of Na+ of > 50 mM become inhibitory except in the presence of I-, which shifts peak binding levels toward higher Na+ concentrations and also elevates the peak binding level. K+ strongly decreased [3H]CFT binding with a shallow inhibition curve, and Na+ could not overcome this effect. Saturation analysis of [3H]CFT binding revealed a single binding site changing its affinity for CFT depending on the concentration of sodium phosphate buffer (6, 10, 30, 50, 130, or 200 mM; 1 mM plus 49 mM NaCl versus 10 mM plus 40 mM NaCl; or 1 mM plus 129 mM NaI versus 10 mM plus 120 mM NaI). No differences were observed in the density of CFT binding sites between any of the conditions examined.

Neurotransmitter transporters: three distinct gene families

Our understanding of the plasma membrane and vesicular transport systems that mediate neurotransmitter re-uptake has been greatly enhanced in the past year by the cloning and characterization of two additional gene families involved in this process, the excitatory amino acid transporters and the vesicular amine transporters. Additional members of the previously defined family of Na+/Cl(-)-dependent transporters continue to be identified.

Effects of several cations on the neuronal uptake of dopamine and the specific binding of [3H]GBR 12783: attempts to characterize the Na+ dependence of the neuronal transport of dopamine

We have studied the effects of several cations on (1) the neuronal uptake of [3H]dopamine ([3H]DA) and (2) the specific binding of 1-[2-(diphenylmethoxy)ethyl]-4-(3-phenyl-2-[1-3H]propenyl)piperazi ne ([3H]GBR 12783) to a site associated with the neuronal carrier of DA, in preparations obtained from rat striatum. When studied under the same experimental conditions, both the uptake of [3H]DA and the binding of [3H]GBR 12783 were similarly impaired by the gradual replacement of NaCl by sucrose. In both processes, no convenient substitute for Na+ was found. Furthermore, potential substitutes of Na+ acted as inhibitors of the uptake with a rank order of potency as follows: K+ = Li+ > or = Cs+ > or = Rb+ > choline+ > Tris+ > sucrose, which was somewhat different from that observed in binding studies, i.e., Cs+ > Rb+ > choline+ > or = K+ > Li+ > Tris+ > sucrose. In the presence of either 36 mM or 136 mM Na+, [3H]DA uptake was optimal with 2 mM Mg2+, 1 mM K+, or 1 mM Ca2+. In contrast, higher concentrations of divalent cations competitively blocked the uptake process. K+ concentrations > 50 mM impaired the specific binding, whereas in the millimolar range of concentrations, K+ noncompetitively inhibited the uptake. Decreasing the Na+ concentration increased the inhibitory effect of K+, Ca2+, and Mg2+ on the specific uptake. An increase in NaCl concentration from 0 to 120 mM elicited a significant decline in the affinity of some substrates for the [3H]GBR 12783 binding site. An uptake study performed using optimal experimental conditions defined in the present study revealed that decreasing Na+ concentration reduces the affinity of DA for the neuronal transport. We propose a hypothetical model for the neuronal transport of DA in which both Na+ and K+ membrane gradients are involved.